Many methods are known in the prior art for monitoring the machining of workpieces.
A method for monitoring the operating state of a machine tool is thus known from DE 10 2005 034 768 A1 in order to diagnose critical states even before damage occurs and thus to avoid the costs and expense caused by damage and unplanned breakdowns. In the known method rotating components of a machine tool, such as rotors of tool spindles or motor spindles, pumps or fans, are monitored by means of a vibration sensor. For this purpose low-frequency vibrations are recorded by the vibration sensor in order to detect imbalances and/or tool vibrations and thus, for example, to detect a poorly balanced, incorrectly tensioned or worn tool. The evaluation is made by means of graphs on the basis of individual values of signal amplitudes at predetermined frequencies. However, an evaluation of this type of individual low-frequency vibrations, as is also known from DE 102 44 426 D4 and DE 103 40 697 A1, is only adapted to a limited extent for the assessment of a chipping process in terms of the quality of the machining of the workpiece.
In order to optimise a chipping process it is known from DE 698 04 982 T2 to record low-frequency vibrations during the machining of the workpiece and, depending on the information regarding the tool, to supply benchmark values for the speed of the tool, with which undesired vibrations which are known as chattering can be eliminated or reduced.
DE 44 05 660 A1 also deals with the reduction or prevention of such a chattering, which is recorded by a vibration sensor, a control mechanism being used to do this.
An arrangement of vibration sensors for obtaining a signal from the machining process is known from D 94 03 901. In this instance a structure-borne sound sensor is fixed to a sensing arm which is in contact with the workpiece, in such a way that sound signals and chattering vibrations generated by the machining process are transferred from the workpiece to the sensor. In this regard D 94 03 901 also mentions high-frequency sound signals. However, the term ‘high-frequency’ is used in conjunction with DE 38 29 825 A1, which feeds a frequency range between 20 kHz and 2 MHz to a mean-value former. Even this frequency range is hardly transferable and recognisable by the sensing arm coupling of D 94 03 901.
A method for assessing machining processes is known from DE 44 36 445 A1, in which method vibrations/structure-borne sound signals of a tool are registered on the one hand under load and on the other hand at the same speed with no load, and a one-dimensional comparison of the corresponding vibration number of the operation with no load and under load is made for each speed in order to assess the tool.
A cutting tool is known from each of WO 88/07911 and WO 89/12528 comprising an integrated sound sensor which supplies a one-dimensional voltage signal, which is proportional to the vibration frequency.
In DE 38 29 825 C2, during chipping of a workpiece the signal level of a sound sensor is registered as a function of the frequency and averaged over time intervals. A comparison of the mean values with threshold or setpoint values makes it possible to draw conclusions regarding the quality of the tool or the machining process.
The drawback that the tool and the chipping process can only be assessed inadequately is inherent to all known methods.
In addition, the known methods are limited to machining by means of chipping.
No reliable sound-based methods for monitoring other machining processes such as welding (laser welding, arc welding, etc.), forming, joining and/or separating or the like are known in the prior art.
For instance, optical systems for monitoring a laser process are thus currently used which measure the light reflected from the site of action and which attempt to derive from the spectrum or intensity how the actual laser process is taken up by the material. It is not therefore always possible to obtain satisfactory results, since a plurality of materials are interconnected and the process of penetration welding, i.e. whether the laser energy also results in the necessary melting and thermal penetration of all components, cannot be checked by the laser emission reflected on the surface.
In addition no reliable sound-based methods for monitoring components in operation are known in the prior art, for example a steel wheel of a train carriage during operation of the train, or a component of an engine during operation. In particular in the case of safety-relevant applications, such as in the transportation of people for example by trains, aircraft and motor vehicles, or in systems which are potentially dangerous, for example power stations, the avoidance of component failures is indispensable and is only possible at high cost by regular inspections and testing outside operation.